The payload capability of a mining truck is the size parameter which counts in final use of such trucks. However, in the design process it is the gross vehicle mass that is controlled by the availability of major components and systems used in these trucks. The tires, engine, transmission system, retardation system, braking system, suspension system and many other components must match the gross vehicle mass rating of the truck. There are very definite limits to the load rating of tires, power rating of engines, etc. and in considering possible future mining truck developments it is necessary to start with the possible future developments of engines, tires, transmission systems and other smaller components. After the possible gross vehicle mass ratings have been established, the likely tare mass values can be established, and subtraction of the tare mass from the gross vehicle mass rating provides the rated payload capacity for a possible design.
There is much commonality between currently available trucks made by different manufacturers and the following observations are generally applicable regardless of the truck manufacturer:
The travel of the rear suspension system of a fully loaded truck is very limited compared to the scale of the truck. Typically the maximum travel in the compression direction of the rear axle relative to the main frame is only of the order of 50 mm when loaded.
Most of the compliance in the rear of the truck is provided by the tires.
The main frames of these trucks are complex welded steel structures that are heavy (e.g. 16.5 tonnes for the main frame of a truck with a payload rating of 172 tonnes), expensive to design, develop and manufacture, and prone to fatigue cracking.
The main load carrying member (the body) of the trucks is a very strong and generally stiff member. This strength and stiffness is a consequence of the need for the body to withstand the shock loads applied during loading of large rocks by large excavators.
The body is generally supported from the main frame of the truck at numerous points. For example at the rear pivot points, at two, four, six or eight points along the underside of the body and in some trucks also at forward extensions of the body which contact the main frame at points which are close to being above the line of the front wheels. This system of supporting the stiff body causes high variations of stress levels in the supporting main frame of the truck as the truck traverses over uneven ground. This feature causes fatigue problems, high fabricating costs and the need for considerable expenditure to limit the uneveness of the ground on which the trucks travel.
The body is tipped (hoisted) by hydraulic cylinders which react against the main frame of the truck at points near to midway between the front and rear wheels. This causes very large bending loads to be applied to the main frame of the truck and requires that the main frame be very massive at the mid sections.
The total width of the four rear tires is large compared to the total width of the truck. Typically 65% of the total width of a truck is taken up by the four rear tires. With present designs of truck this leads to a very narrow main frame for the truck and very high bending loads on the rear axle and wheel support systems. The narrow main frame causes shortage of space for maintenance of some components, high stress changes during cornering manoeuvres and design restrictions on the body. The net effect is high weight and cost for the main frame, the rear axle, the wheel support assemblies and the body.
The dual rear tires are rotationally locked together. During short radius turning manoeuvres (frequent occurrences in typical mining operations), this causes severe scrubbing type wear of the tires due to the differential travel distance effect. There is also a need for considerable care in matching tire outside diameters and inflation pressures to minimise differential rolling radius effects for straight ahead driving. Relative scrubbing between the two tires of a dual set is considered to contribute significantly to total wear of rear tires on large mining trucks.
In general with currently available truck designs, the transfer of forces between the body and the tires is through a very indirect path which involves high bending loads in the main frame of the truck, the rear axle housing and the rear wheel support systems (the final drives or the wheel motor housings). Furthermore these bending loads fluctuate greatly as the truck travels over uneven ground.
A typical very large mining truck is shown in FIG. 1 of the drawings and it will be noted that the frame structure is quite substantial and this results from the frame being required to bear the load supported by the body of the truck by contact between the body and the upper surfaces of the frame, and by virtue of the body hoisting rams being connected to the frames as shown.
While many truck frame design improvements have been suggested over the years, no one design has addressed more than a few of the difficulties which have been outlined above. For example, U.S. Pat. No. 3,704,040 Davis et al discloses a frame arrangement in which the rear wheel pairs are centrally supported, the independent suspension arrangement which is described as addressing many of the problems created by uneven terrain is extremely complex and consequently expensive, and most of the load supported by the body is transferred indirectly through the frame, resulting in a frame of substantially proportions and mass.